In an image forming apparatus using the electrophotographic method, the single-component developing method using toner alone and the two-component developing method using both toner and carrier have been known as a development method for developing an electrostatic latent image formed on an image carrier.
In the single-component developing method, generally, toner is made to pass through a regulating section formed by a toner carrier and a regulating plate pressed against the toner carrier, whereby the toner is charged and a desired thin toner layer is obtained. This method has advantages of simplification, downsizing and cost reduction of the apparatus.
In the meantime, toner deterioration tends to be accelerated by the heavy stress at the regulating section, and toner charge-acceptance ability tends to be reduced. Further, the regulating member as a charge providing member for providing charge to the toner and the surface of the toner carrier are contaminated with the toner or external additive agent, whereby the charge-providing ability for providing charge to the toner is also reduced. This will reduce the amount of toner charge and will cause fogging and related problems, with the result that the service life of the development device is reduced.
Comparison reveals that, the two-component developing method is advantageous to realize a longer service life since the toner is mixed with a carrier to be charged by triboelectric charging, thereby causing less stress, and since the carrier is not easily contaminated with toner or external additives because of a greater area of its surface.
However, in the two-component developing method, when an electrostatic latent image on the image carrier is to be developed, the image carrier surface is brushed by a magnetic brush formed of the developer. This may create a problem that a mark of the magnetic brush remains on a developed image. Further, the carrier tends to be attached to the image carrier, whereby an image defect occurs.
The so-called hybrid development method was disclosed (e.g., Japanese Patent Application Publication No. H05-150636) as a development method that provides image quality as high as that of the single-component developing method, and solves the problem of image defect, and this method is characterized by a long service life achieved by the two-component developing method using a two-component developer. In this hybrid development method, a two-component developer is carried on the developer carrier, and only the toner is supplied from the two-component developer to the toner carrier.
However, the hybrid development method described in the Japanese Patent Application Publication No. H05-150636 includes such problems as reduction in density at the time of high-speed development or occurrence of development hysteresis (ghost).
The reduction in density at the time of high-speed development refers to the problem wherein, when high-speed image formation has been practiced, the toner traveling speed cannot catch up with the development nip time, with the result that image density is reduced.
The usual one-component development method has been used only in the range of a low speed, since it has a problem, which is also found in the non-contact one-component development, that a heavy stress is applied to the toner, thereby causing heat and fusion of toner at the regulating section. For that reason, that problem has not been thought to be a big problem. The hybrid development method is free of these problems, and realizes image forming at a considerably high speed. For example, in an apparatus wherein the system speed exceeds 500 mm/s, the aforementioned problem does not arise.
The problem of development hysteresis (ghost) commonly occurs to the hybrid development method. The residual toner remaining on the toner carrier without being used for development appears on the image as development hysteresis (ghost) in the next development process.
In a portion (supply area) where the developer carrier for supplying toner to the toner carrier faces the toner carrier, the toner to be used for development is supplied, and collection of the residual toner is performed in the same opposing portion between the developer carrier and the toner carrier. In this case, bias is applied in the direction of supplying toner. This causes difficulties in collecting the residual toner, hence, insufficiency in collection capability. Thus, a greater amount of post-development residual toner is found in some areas while a smaller amount of post-development residual toner is observed in other areas. This difference in residual toner appears as the contrast of density in the next development process.
One of the techniques for solving the problem of reduction in density in the high-speed development mode is disclosed in the Japanese Patent Application Publication No. 2005-37523 and others where a plurality of toner carriers are provided to save time for the total development nip time required for jumping of the toner, whereby the toner density is ensured.
In the structure described in the Japanese Patent Application Publication No. 2005-37523, the toner jumps more than once. This allows a toner image to be positively formed on the photoreceptor, and suppresses the reduction in density of the toner image cause by high speed operation, even when the photoreceptor is rotated at a high speed. This structure reduces the amount of toner carried on each toner carrier compared to a structure having only one toner carrier. This reduces the density contrast between the portion where toner has been used for development and the portion wherein toner has not been used for development. Thus, occurrence of a ghost is kept to a considerably small degree, as disclosed.
However, the study made by the present inventors shows that the structure disclosed in the Japanese Patent Application Publication No. 2005-37523 fails to ensure a sufficient capability of collecting from the toner carrier the post-development residual toner. Thus, in the next toner supplying step, toner is supplied to the toner carrier having unevenness of toner density. Therefore, the contrast of density can be reduced to a certain extent but cannot be sufficiently reduced, and the ghost cannot be completely eliminated.